In The discovery of a new world, or, A discourse tending to prove, that ('tis probable) there may be another habitable world in the moon (1638) he argued that celestial bodies were not made of some incorruptible matter qualitatively different to that of the earth and that they did not revolve on crystal spheres. He hypothesised that the moon was a world like our own and possibly inhabited. In later editions of this work he even speculated that it might be possible to visit the Moon in ‘a Flying-Chariot; in which a Man may sit’ and colonize it.

In 1640 he supplemented Discovery with a second part entitled Discourse concerning a new planet; tending to prove, that ('tis probable) our earth is one of the planets. Drawing heavily on contemporary literature on the subject, particularly the works of Galileo, Discourse illustrates and defends the heliocentric system of Copernicus and tries to show how the earth’s movement around the sun might be possible. Reprinted several times in the seventeenth and early eighteenth century, these works were hugely influential and helped gain acceptance of the ideas of Copernicus, Kepler and Galileo in England.